Electrode configuration for piano MEMs micromirror

ABSTRACT

A micro-electro-mechanical (MEMs) mirror device for use in an optical switch is disclosed. A “piano”-style MEMs device includes an elongated platform pivotally mounted proximate the middle thereof by a torsional hinge. The middle portion of the platform and the torsional hinge have a combined width less than the width of the rest of the platform, whereby several of these “piano” MEMs devices can be positioned adjacent each other pivotally mounted about the same axis with only a relatively small air gap therebetween. In a preferred embodiment of the present invention specially designed for wavelength switching applications, a greater range of arcuate motion for a mirror mounted thereon is provided by enabling the platform to rotate about two perpendicular axes. The MEMs mirror device according to the preferred embodiment of the present invention enables the mirror to tilt about two perpendicular axes, by the use of an “internal” gimbal ring construction, which ensures that a plurality of adjacent mirror devices have a high fill factor, without having to rely on complicated and costly manufacturing processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/850,407 filed May 21, 2004 which is a continuation-in-part ofU.S. patent application Ser. No. 10/445,360 filed May 27, 2003 now U.S.Pat. No. 6,934,439, which claims priority from U.S. Patent ApplicationNo. 60/383,106 filed May 28, 2002, which are both incorporated herein byreference. The present application also claims priority from U.S. patentapplications Ser. Nos. 60/504,210 filed Sep. 22, 2003; 60/537,012 filedJan. 20, 2004; and 60/558,563 filed Apr. 2, 2004, which are allincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a micro-electro-mechanical (MEMs)mirror device for use in an optical switch, and in particular to anelectrode arrangement for a 2d gimbal ring MEMs mirror device.

BACKGROUND OF THE INVENTION

Conventional MEMs mirrors for use in optical switches, such as the onedisclosed in U.S. Pat. No. 6,535,319 issued Mar. 18, 2003 to Buzzetta etal, to redirect beams of light to one of a plurality of output portsinclude an electro-statically controlled mirror pivotable about a singleaxis. Tilting MEMs mirrors, such as the ones disclosed in U.S. Pat. No.6,491,404 issued Dec. 10, 2002 in the name of Edward Hill, and UnitedStates Patent Publication No. 2003/0052569, published Mar. 20, 2003 inthe name of Dhuler et al, which are incorporated herein by reference,comprise a mirror pivotable about a central longitudinal axis, and apair of electrodes, one on each side of the central longitudinal axisfor actuating the mirror. The Dhuler et al reference discloses thepositioning of electrodes at an angle to the mirrored platform toimprove the relationship between the force applied to the mirror and thegap between the mirror and the electrodes. The MEMs mirror device,disclosed in the aforementioned Hill patent, is illustrated in FIG. 1,and includes a rectangular planar surface 2 pivotally mounted bytorsional hinges 4 and 5 to anchor posts 7 and 8, respectively, above asubstrate 9. The torsional hinges may take the form of serpentinehinges, which are disclosed in U.S. Pat. No. 6,327,855 issued Dec. 11,2001 in the name of Hill et al, and in United States Patent PublicationNo. 2002/0126455 published Sep. 12, 2002 in the name of Robert Wood,which are incorporated herein by reference. In order to positionconventional MEMs mirror devices in close proximity, i.e. with a highfill factor, fill factor=width/pitch, they must be positioned with theiraxes of rotation parallel to each other. Unfortunately, this mirrorconstruction restraint greatly restricts other design choices that haveto be made in building the overall switch.

When using a conventional MEMs arrangement, the mirror 1 positioned onthe planar surface 2 can be rotated through positive and negativeangles, e.g. ±2°, by attracting one side 10 a or the other side 10 b ofthe planar surface 2 to the substrate 6. Unfortunately, when the deviceis switched between ports at the extremes of the devices rotationalpath, the intermediate ports receive light for fractions of amillisecond as the mirror 1 sweeps the optical beam past these ports,thereby causing undesirable optical transient or dynamic cross-talk.

One solution to the problem of dynamic cross-talk is to initially orsimultaneously rotate the mirror about a second axis, thereby avoidingthe intermediate ports. An example of a MEMs mirror device pivotableabout two axes is illustrated in FIG. 2, and includes a mirror platform11 pivotally mounted by a first pair of torsion springs 12 and 13 to anexternal gimbal ring 14, which is in turn pivotally mounted to asubstrate 16 by a second pair of torsion springs 17 and 18. Examples ofexternal gimbal devices are disclosed in U.S. Pat. No. 6,529,652 issuedMar. 4, 2003 to Brenner, and U.S. Pat. No. 6,454,421 issued Sep. 24,2002 to Yu et al. Unfortunately, an external gimbal ring greatly limitsthe number of mirrors that can be arranged in a given area and therelative proximity thereof, i.e. the fill factor. Moreover, the externalgimbal ring may cause unwanted reflections from light reflecting off thesupport frame. These references also require at least four electrodes toactuate each mirror.

Another proposed solution to the problem uses high fill factor mirrors,such as the ones disclosed in U.S. Pat. No. 6,533,947 issued Mar. 18,2003 to Nasiri et al, which include hinges hidden beneath the mirrorplatform. Unfortunately, these types of mirror devices require costlymulti-step fabrication processes, which increase costs and result in lowyields, and rely on four different pairs of electrodes for actuation.

An object of the present invention is to overcome the shortcomings ofthe prior art by providing a MEMs mirror device that can pivot aboutperpendicular axes using a limited number of electrodes.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to a micro-electro-mechanicaldevice mounted on a substrate comprising:

-   -   a pivoting member pivotally mounted on said substrate about        first and second axes, said pivoting member including a first        and a second supporting region on opposite sides of said first        axis;    -   a first hinge extending from said substrate rotatable about the        first axis;    -   a gimbal ring surrounding said first hinge;    -   a second hinge extending from said gimbal ring to said pivoting        member rotatable about the second axis;    -   a first electrode beneath the first supporting region for        pivoting the pivoting member about the first axis;    -   a second electrode beneath the second supporting region for        pivoting the pivoting member about the first axis; and    -   a third electrode beneath a portion of said pivoting member        along the first axis, adjacent said second hinge.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to theaccompanying drawings which represent preferred embodiments thereof,wherein:

FIG. 1 is an isometric view of a conventional tilting MEMs mirrordevice;

FIG. 2 is a plan view of a pair of conventional external gimbal ringMEMs mirror devices;

FIG. 3 is an isometric view of a plurality of Piano-MEMs mirror devices;

FIG. 4 is an isometric view of a hinge structure of the mirror devicesof FIG. 3;

FIG. 5 is an isometric view of an electrode structure of the mirrordevices of FIG. 3;

FIG. 6 is an isometric view of a plurality of Piano-MEMs mirror devicesaccording to an alternative embodiment of the present invention withelectrode shields, light redirecting cusps, and a raised ground plane;

FIG. 7 is an isometric view of a plurality of Piano-MEMs mirror devicesaccording to an alternative embodiment of the present invention withelectrode shields;

FIG. 8 is a plan view of a pair of internal gimbal ring MEMs mirrordevices according to the present invention;

FIG. 9 is an isometric view of an internal gimbal ring MEMs mirrordevice according to the present invention;

FIG. 10 is an isometric view of an alternative embodiment of theinternal gimbal ring MEMs mirror devices according to the presentinvention;

FIG. 11 is an isometric view of a hinge structure of the mirror devicesof FIG. 9;

FIG. 12 is an isometric view of an electrode structure of the mirrordevices of FIGS. 9 and 10;

FIG. 13 is a graph of Voltage vs Time provided by the electrodestructure of FIG. 11;

FIG. 14 is an isometric view of internal gimbal ring MEMs mirror devicesutilizing a three electrode arrangement according to the presentinvention;

FIG. 15 is an isometric view of the three electrode arrangement of FIG.14;

FIG. 16 is a plan view of an ideal placement of the three electrodes ofFIGS. 14 and 15 relative to the pivoting platform;

FIG. 17 is a plan view of a possible misalignment of the threeelectrodes of FIGS. 14 and 15;

FIG. 18 is a plan view of another possible misalignment of the threeelectrodes of FIGS. 14 and 15;

FIG. 19 is a graph of Voltage vs Time for the three electrodes of FIGS.14 and 15;

FIG. 20 is an isometric view of another embodiment of the presentinvention with an offset section on the pivoting member;

FIG. 21 is a plan view of the embodiment of FIG. 20;

FIG. 22 is an end view of the embodiment of FIGS. 20 and 21;

FIG. 23 is a schematic diagram of a wavelength switch utilizing themirror devices of the present invention;

FIG. 24 is a schematic diagram of an input/output assembly for thewavelength switch of FIG. 23; and

FIG. 25 is a schematic diagram of an alternative embodiment of an inputassembly for the wavelength switch of FIG. 23.

DETAILED DESCRIPTION

An array of “Piano” MEMs mirror devices 21, 22 and 23, which pivot abouta single axis of rotation θ_(y) above a substrate 25, is illustrated inFIGS. 3, 4 and 5. Each mirror device 21, 22 and 23 includes a pivotingmember or platform 26 defined by first and secondsubstantially-rectangular planar supporting regions 27 and 28 joined bya relatively-thin substantially-rectangular brace 29 extendingtherebetween. Typically, each planar surface is coated with a reflectivecoating, e.g. gold, for simultaneously reflecting a pair of sub-beams oflight traveling along parallel paths, as will be hereinafter discussed.Each brace 29 acts like a lever and is pivotally mounted to anchor posts30 and 31 via first and second torsional hinges 32 and 33, respectively.The anchor posts 30 and 31 extend upwardly from the substrate 25. Theends of the first torsional hinge 32 are connected to the anchor post 30and the brace 29 along the axis θ_(y). Similarly, the ends of the secondtorsional hinge 32 are connected to the anchor post 31 and the brace 29along the axis θ_(y). Preferably, each of the first and second torsionalhinges 32 and 33 comprises a serpentine hinge, which are considerablymore robust than conventional torsional beam hinges. The serpentinehinge is effectively longer than a normal torsional hinge, which spansthe same distance, thereby providing greater deflection and strength,without requiring the space that would be needed to extend a normalfull-length torsional hinge.

With particular reference to FIG. 5, each platform 26 is rotated by theselective activation of a first electrode 36, which electro-staticallyattracts the first planar section 27 theretowards or by the selectiveactivation of a second electrode 37, which electro-statically attractsthe second planar section 28 theretowards. A gap 38 is provided betweenthe first and second electrodes 36 and 37 for receiving the anchor posts31, which extend from the substrate 25 to adjacent the platforms 26.

In the disclosed open loop configuration, the angular position of theplatforms 26 depend non-linearly on the voltage applied by theelectrodes 36 (or 37), i.e. as the applied voltage is increasedlinearly, the incremental change in angular platform position is greateras the voltage increases. Accordingly, there is a maximum voltage, i.e.an angular platform position, at which the platform angular positionbecomes unstable and will uncontrollably tilt until hitting part of thelower structure, e.g. the electrode 36. This maximum voltage sets therange of angular motion that the platform 26 can travel. The instabilityin the platform's angular position is a result of the distance betweenthe platform 26 and the electrode 36 (the hot electrode) decreasing morerapidly at the outer free ends of the platform 26 than at the innersections, nearer the pivot axis θ_(y). As a result, the force per unitlength along the platform 26 increases more rapidly at the outer freeends of the platform 26 than the inner sections. To increase theplatform's range of angular motion, the field strength, i.e. the forceper unit area, that is sensed at the outer free ends of the platform 26must be reduced. With reference to FIG. 5, this is accomplished byproviding the electrodes 36 and 37 with a two-step configuration. Uppersteps 36 a and 37 a are positioned proximate the inner end of theplatform 26, i.e. the Y axis, while lower steps 36 b and 37 b arepositioned under the outer free ends of the platform 26, thereby makingthe gap between the platforms 26 and the electrodes 36 and 37 greater atthe outer free end than the inner end. The area of the lower steps 36 band 37 b can also be made smaller, thereby reducing the force per unitarea sensed by the outer free end of the platform 26. Multi-stepelectrodes, e.g. three or more can also provide a more even distributionof force.

A consequence of closely packed micro-mirrors is that the actuation of asingle mirror will impart a torque, i.e. an angular rotation, ontoadjacent mirrors as a result of fringing electric fields. In an effortto minimize this cross-talk, electrode grounding shields 41 arepositioned on the substrate 25 around or on either side of the first andsecond electrodes 36 and 37 forming electrode cavities, which areelectrically isolated from each other. FIG. 5 illustrates C-shapedgrounding shields 41, which include lateral portions 41 a for partiallysurrounding the first and second electrodes 36 and 37. The groundingshields 41 are kept at ground potential, i.e. the same as the mirroredplatforms 26, while one of the first and second electrodes is held at anactivation voltage, e.g. 100 Volts.

Trace lines 36 c and 37 c electrically connect the electrodes 36 and 37,respectively, to a voltage supply (not shown). Since the trace lines 36c and 37 c also act as a hot electrode, i.e. contributing to the totaltorque applied to the platform 26, covering the traces 36 c and 37 cwith a ground plane 43 (FIG. 6) also reduces the force applied to theouter free end of the platform 26.

FIG. 6 also illustrates an alternative configuration for the electrode36, in which the two step hot electrode 36 is sunken slightly below asurrounding grounded metallic surface, which is a continuation of theground plane 43. A small vertical step 44 between the hot electrode 36and the surrounding ground plane 43 is a dielectric surface thatisolates the hot electrode 36 from the surrounding ground plane 43. Thisarrangement reduces the angular drift of the platform 26, which iscaused by a build up of electrostatic charges on exposed dielectric orinsulating surfaces. The electric field generated by these electrostaticcharges perturbs the electric field generated by the applied voltagefrom the electrodes 36 and 37, thereby causing the angular position ofthe platform 26 to drift over time. The present arrangement limits theexposed dielectric to the small vertical surface 44, which generateselectrostatic field lines that do not significantly affect the fieldlines between the hot electrodes 36 and 37 and the ground plane 43. Tofurther reduce the angular drift of the platform 26, the verticalsurface 44 can be under cut beneath the ground plane 43 at a slightnegative angle ensuring that the gap between the hot electrode 36 andthe ground plane 43 is substantially zero. The ground plane 43 couldalso be positioned slightly below the hot electrodes 36 and 37 to createthe vertical step.

Since the MEMs mirror devices 21, 22 and 23 are for use in opticaldevices, i.e. wavelength blockers and multiple wavelength switches (seeFIG. 23), which include a grating for dispersing the light into spectralwavelength component channels, it is an important performancerequirement that the spectral response has a high rejection of lightbetween the selected wavelength channels. Unfortunately, in conventionalMEMs devices, light passes between the mirrors and is reflected off thesubstrate back into the optical device, thereby leading to adeterioration in the isolation between the wavelength channels.Accordingly, the present invention provides back reflection cusps 50,defined by angled, curved or concave reflecting surfaces intersectingalong a ridge, extending longitudinally below the gap between theplatforms 26, for scattering any light passing between the mirroredplatforms 26 in a direction substantially parallel to the surface of theplatforms 26.

To further eliminate cross-talk between adjacent electrodes, additionalplatform shields 42 (FIG. 7) can be added to the underside of the planarsupporting regions 27 and 28, outside or inside of the electrode shields41. Typically, in the rest position, the two different sets of shields41 and 42 do not overlap; however, as the platform 26 tilts the platformshields 42 begin to overlap the grounding shielding 41. The addedprotection provided by overlapping shielding is particularlyadvantageous, when the tilt angle of the platform 26 is proportional tothe voltage applied to the electrode 36 (or 37), such as in open loopconfigurations. Accordingly, the greater the tilt angle, the greater therequired voltage, and the greater the amount of potential cross-talk,but consequently the greater the amount of shielding provided by theoverlapping ground and platform shields 41 and 42, respectively. Backreflection cusps 50 are also provided for reasons hereinbeforediscussed. A single structure 50 between adjacent electrodes can replacethe pair of adjacent shields 41.

With reference to FIG. 8, a pair of internal gimbal ring MEMs mirrordevices 131 and 132 are illustrated mounted adjacent each other on asubstrate 133. The present invention enables mirrors 134 and 135 to bepositioned relatively close together, i.e. with a high fill factor,while still providing the two degrees of motion provided by the morecomplicated prior art.

With further reference to FIG. 9, a first torsion hinge 137, preferablyin the form of a rectangular beam, is fixed, proximate the middlethereof, to the substrate 133 via a central anchor post 138. Thesupporting structure for the mirror device of the present invention isbased on a single anchor post 138, rather than the dual anchor pointsrequired in the aforementioned external gimbal ring devices. The firsttorsion hinge 137 provides for rotation θ_(y) about a first axis Y, andmay also include a serpentine hinge 140, as illustrated in mirror device131, or any other torsional hinge known in the art. Opposite sides of aninternal gimbal ring 139 are connected to opposite ends of the firsttorsion hinge 137, whereby the first torsion hinge 137 bisects theinternal gimbal ring 139. The internal gimbal ring 139 is preferably notflexible, but can take various geometric forms, although rectangular orcircular frames would be the most convenient to fabricate and use.Spring arms 141 and 142, which define a second torsion hinge, extendoutwardly from opposite sides of the internal gimbal ring 139perpendicular to the first torsion hinge 137. Each of the spring armsmay also include a serpentine hinge as hereinbefore described. Thesecond torsion hinge provides for rotation θ_(x) about a second axis X,which is perpendicular to the first axis Y, but still substantially inthe same plane as the mirrors 134 and 135. A generally rectangularplatform 143, for supporting one of the mirrors 134 or 135, is mountedon the ends of the spring arms 141 and 142. Preferably, the platform 143is comprised of a pair of rectangular planar surfaces 144 and 145 joinedtogether by a pair of elongated braces 147 and 148, which extend oneither side of the internal gimbal ring 139 parallel with the springarms 141 and 142.

Fabrication of the preferred embodiment illustrated in FIGS. 8 and 9, issimplified by having all of the structural elements, i.e. the firsttorsional hinge 137, the gimbal ring 139, the spring arms 141 and 142,and the first and second planar surfaces 144 and 145, in the same uppersubstrate layer and having coplanar upper surfaces, whereby the samebasic process steps are used as are used to fabricate the MEMs deviceillustrated in FIG. 1. In particular, a single photolithographic step isused to identify the structural elements, followed by a deep reactiveion etching (DRIE) step used to remove the unwanted portions of theupper substrate. Finally the moveable elements in the upper substrateare released from the lower substrate by removal of a sacrificial layertherebetween.

FIGS. 10 and 11 illustrate an array of internal gimbal ring MEMs mirrordevices 201 utilizing a first pair of serpentine torsional hinges 202for pivoting a rectangular platform 203, including first and secondplanar supporting regions 203 a and 203 b, about a first axis ofrotation θ_(x), and a second pair of serpentine torsional hinges 204 forrotating the platform 203 about a second axis of rotation θ_(y) above abase substrate 205. The first pair of serpentine torsional hinges 202extend from a single anchor post 206, which extends upwardly from thebase substrate 205 through the center of the platform 203, i.e. at theintersection of the minor and major axes thereof. Outer ends of thefirst pair of torsional serpentine torsional hinges 202 are connected toa rectangular gimbal ring 208, which surrounds the first pair ofserpentine hinges 202, at points along the minor axes (θ_(y)) of theplatform 203. The second pair of serpentine torsional hinges 204 extendfrom opposite sides of the gimbal ring 208 into contact with theplatform 203, at points along the major axis (θ_(x)) of the platform203.

To provide a full range of motion for the platform 143 or 203, a set offour two-step electrodes 211, 212, 213 and 214 are provided (See FIG.12); however, for the present invention only the first, second and thirdelectrodes 211, 212 and 213 are required to roll the mirrors out ofalignment with any intermediate output ports and then back intoalignment with a designated output port. As in FIG. 5, each of theelectrodes 211, 212, 213 and 214 include an upper step 211 a, 212 a, 213a, and 214 a, and a lower step 211 b, 212 b, 213 b, 214 b, respectively,for reasons discussed hereinbefore. Accordingly, first, second and thirdvoltages can be established between the platform 143 or 203 and thefirst electrode 211, the second electrode 212 and the third electrode213, respectively. Initially, the first and second electrodes 211 and212 are activated to rotate the platform 143 or 203 about θ_(x).Subsequently, the first voltage is gradually lowered to zero, while thethird voltage is gradually increased until it is equivalent to thesecond voltage (See FIG. 13). To minimize unwanted effected caused byringing, i.e. vibration of the mirrors caused by an abrupt start orstop, the first, second and third voltages are increased and decreasedgradually, e.g. exponentially, as evidenced in FIG. 13, whichillustrates the voltages curves for the various electrodes (first,second and third) over the actuation time of the mirror device. Variousmirror tilting patterns can be designed based on the desiredcharacteristics, e.g. attenuation, of the light.

An improved electrode configuration is illustrated in FIGS. 14 and 15,in which a first two-step θ_(y) electrode 236 includes an upper U-shapedstep 236 a, and a lower rectangular step 236 b. The arms of the U-shapedstep 236 a extend from the lower step 236 b on opposite sides of thesecond hinge 204 beneath the first planar supporting region 203 a.Similarly, a second two-step θ_(y) electrode 237 includes an upperU-shaped step 237 a, and a lower rectangular step 237 b. The arms of theU-shaped step 237 a extend from the lower rectangular step 237 b onopposite sides of the second hinge 204 beneath the second planarsupporting region 203 b. A single two-step θ_(x) electrode 238 includesan upper U-shaped step 238 a, and lower rectangular steps 238 bextending from each arm of the upper U-shaped step. The single θ_(x)electrode 238 extends from adjacent the first θ_(y) electrode 236 toadjacent the second θ_(y) electrode 237 across the gap therebetween, andbeneath one side of both the first and second planar supporting regions203 a and 203 b. The lower steps 238 b provide a larger gap between theouter free ends of the platform 203, when the platform is tilted towardsthe first or the second θ_(y) electrode 236 or 237. The arms of theupper U-shaped step 238 b extend on opposite sides of the first pair ofhinges 202. The arms of the U-shaped step 238 a are three to five timeswider than the arms of the U-shaped step 236 a or 237 a. Multi-stepelectrodes are also possible to further spread the application of forceover the length of the platform 203. Actuation of the electrodes iscontrolled by an electrode control 240, as will be discussed hereinafterwith reference to FIG. 19.

An unfortunate consequence of relying on only three electrodes is that aslight misalignment in positioning the platform 203 over the first andsecond θ_(y) electrodes 236 and 237 can result in an imbalance that cannot be corrected for using the single θ_(x) electrode 238. FIG. 16illustrates the ideal case, in which the longitudinal axis of the firstelectrode 236 is aligned with the longitudinal axis X of the platform203. However, FIG. 17 illustrates the results of a mask misalignmentduring fabrication, in which the longitudinal axis X of the platform 203has a +Δx misalignment relative to the electrode axis. Accordingly,actuation of the first θ_(y) electrode 236 would introduce anundesirable tilt in the platform 203 towards the bottom left handcorner, which could not be compensated by the single θ_(x) electrode238. In FIG. 18, the illustrated mask misalignment, in which thelongitudinal axis X of the platform 203 has a −Δx misalignment relativeto the electrode axis. In this case, actuation of the first θ_(y)electrode 236 would introduce an undesirable tilt in the platform 203towards the top left hand corner. However, this tilt can be compensatedfor by applying a voltage to the single θ_(x) electrode 238.Accordingly, the solution to the problem of mask misalignment is tointroduce an intentional or predetermined −Δx misalignment, which wouldcancel or at least minimize any +Δx misalignment and which could becompensated for by the single θ_(x) electrode 238.

FIG. 19 illustrates an electrode voltage vs time graph, detailing thevoltages of the three electrodes 236, 237 and 238 as the platform 203 isswitched from one position to another by an electrode control, i.e. fromreflecting light from one port to another, without traveling directly,i.e. without reflecting light into any intermediate ports. To preventundesirable “ringing” of the platform 203, the voltage VyR of the firstθ_(y) electrode 236 is gradually decreased as the voltage Vx of thesingle θ_(x) electrode 238 is increased. As the voltage VyR decreases tozero, the voltage VyL of the second θ_(y) electrode 237 graduallyincreases. As the voltage VyL reaches its set amount to maintain theplatform in the desired position, the voltage Vx is decreased to aminimum amount, assuming no compensation voltages are required.

When the size of the platform 203 is decreased, e.g. for small pitchmicro-mirrors in the order of <100 um, the electrodes 236, 237 and 238must also be constructed correspondingly smaller. However, due to thefact that the electrodes necessarily become thinner, while sharing thesame mirror section, stable tilt angles are difficult to achieve at highresonant frequencies. Accordingly, the size requirement of theelectrode, and the required electrode spacing become the limiting factorin determining the maximum fill factor. An alternative embodiment of athree electrode configuration is illustrated in FIGS. 20 and 21, inwhich platform 253 are made smaller than the original platforms 203 withfirst and second two-step θ_(y) electrodes 256 and 257 positionedtherebelow. A single θ_(x) electrode 258 is positioned below each offsetsection 259, which extends from the side of the platforms 253 adjacentthe mid-section thereof, i.e. the area of first and second hinges 251and 252. This arrangement enables the single θ_(x) electrode 258 to beseparated from the other two electrodes 256 and 257, and therefore, belarger in size, which enables the electrostatic torque to be increasedfor a common voltage. The added separation between the electrodes 256,257 and 258 minimizes the angular instabilities, when the single θ_(x)electrode 238 is actuated, and reduces the amount of electrical x-talk.Preferably, the two-step θ_(x) electrodes 256 and 257 include the groundplane arrangement as disclosed in FIG. 6 with hot electrodes sunkenrelative to a surrounding ground plane, and only a vertically extendingdielectric layer, which provides a substantially zero-width vertical gapbetween the hot electrode and the ground plane.

For high fill factor applications, a first planar section 253 a of oneplatform 253 is positioned beside a second planar section 253 b of anadjacent platform 253, as in FIGS. 20 and 21, whereby adjacent mirrorshave offset x axes X₁ and X₂, and every other platform pivots about thesame x axis. Only the relatively closely disposed planar sections wouldrequire reflective material thereon, and reflective cusps 260 would onlybe required therebelow (FIG. 22).

Substrate-mounted, grounded cross-talk shields 261 and platform mountedcross-talk shields 262 are provided to further minimize the amount ofelectrical cross-talk between adjacent mirrors. The platform mountcross-talk shields 262 are preferably mounted outside of the platformmounted cross-talk shields 262 with enough spacing to enable rotationabout both the x and the y axis; however, any combination for offsettingthe shields 261 and 262 is possible.

The “piano” MEMs mirror devices according to the present invention areparticularly useful in a wavelength switch 301 illustrated in FIGS. 23,24 and 25. In operation, a beam of light with a plurality of differentwavelength channels is launched via an input/output assembly 302, whichcomprises a plurality of input/output ports, e.g. first, second, thirdand fourth input/output ports 303, 304, 305 and 306, respectively. Thebeam is directed to an element having optical power, such as concavemirror 309, which redirects the beam to a dispersive element 311, e.g. aBragg grating. The dispersive element separates the beam into thedistinct wavelength channels (λ₁, λ₂, λ₃), which are again directed toan element having optical power, e.g. the concave mirror 309. Theconcave mirror 309 redirects the various wavelength channels to an arrayof “piano” MEMs mirror devices 312 according to the present invention,which are independently controlled to direct the various wavelengthchannels back to whichever input/output port is desired. Wavelengthchannels designated for the same port are reflected back off the concavemirror 309 to the dispersive element 311 for recombination andredirection off the concave mirror 309 to the desired input/output port.The concave mirror 309 can be replaced by a single lens with otherelements of the switch on either side thereof or by a pair of lenseswith the dispersive element 311 therebetween.

With particular reference to FIG. 24, the input/output assembly 302includes a plurality of input/output fibers 313 a to 313 d with acorresponding collimating lens 314 a to 314 d. A single lens 316 is usedto convert a spatial offset between the input/output ports into anangular offset. FIG. 25 illustrates a preferred embodiment of theinput/output assembly, in which the unwanted effects of polarizationdiversity are eliminated by the use of a birefringent crystal 317 and awaveplate 318. For incoming beams, the lens 316 directs each beamthrough the birefringent crystal 317, which separates the beam into twoorthogonally polarized sub-beams (o and e). The half waveplate 318 ispositioned in the path of one of the sub-beams for rotating thepolarization thereof by 90°, so that both of the sub-beams have the samepolarization for transmission into the remainder of the switch.Alternatively, the waveplate 318 is a quarter waveplate and rotates oneof the sub-beams by 45° in one direction, while another quarterwaveplate 319 rotates the other sub-beam by 45° in the oppositedirection, whereby both sub-beams have the same polarization. Foroutgoing light, the polarization of one (or both) of the similarlypolarized sub-beams are rotated by the waveplate(s) 318 (and 319), sothat the sub-beams become orthogonally polarized. The orthogonallypolarized sub-beams are then recombined by the birefringent crystal 317and output the appropriate input/output port. Themicro-electro-mechanical devices according to the present invention areparticularly well suited for use in switching devices with polarizationdiversity front ends, since they provide a pair of reflecting surfaces,i.e. one for each sub-beam.

1. A method of rotating a micro electro-mechanical (MEMs) device about afirst axis from a first tilted position to a second tilted position,while bypassing a third position therebetween, comprising the steps of:a) providing a pivoting member pivotally mounted above a substrate aboutthe first axis and a second axis; b) providing first and secondelectrodes on opposite sides of the first axis underneath the pivotingmember for pivoting the first pivoting member about the first axis; c)providing a third electrode along the first axis offset from the secondaxis for pivoting the pivoting member about the second axis; d)gradually decreasing voltage to the first electrode, while graduallyincreasing voltage to the third electrode for rotating the pivotingmember about the first axis away from the first position, and forrotating the pivoting member about the second axis, respectively; and e)gradually decreasing voltage to the third electrode, while graduallyincreasing voltage to the second electrode for rotating the pivotingmember about the second axis, and for rotating the pivoting member aboutthe first axis, respectively, to the second position.
 2. The methodaccording to claim 1, wherein step c) includes extending the thirdelectrode from adjacent the first electrode to adjacent the secondelectrode beneath the pivoting member.
 3. The method according to claim2, wherein step c) includes providing shields between the first andthird electrodes, and between the second and third electrodes forlimiting the amount of electrical cross-talk therebetween.
 4. The methodaccording to claim 3, wherein step c) includes providing the shieldsextending upwardly from the substrate.
 5. The method according to claim3, wherein step c) further includes providing the shields extendingdownwardly from the pivoting member.
 6. The method according to claim 3,wherein step c) includes providing the shields extending downwardly fromthe pivoting member and upwardly from the substrate.
 7. The methodaccording to claim 1, wherein step b) includes offsetting a longitudinalaxis of said first and second electrodes from a longitudinal axis ofsaid pivoting member on an opposite side of said third electrode toensure any mask misalignment of said pivoting member and said first andsecond electrodes can be compensated by said third electrode.
 8. Themethod according to claim 1, wherein step a) includes providing thepivoting member with first and second supporting regions extending fromopposite sides of first and second hinges, which enable rotation aboutthe first and second axes, respectively; and wherein said first andsecond hinges are positioned between said first and second supportingregions, whereby outer edges of said pivoting member are free of saidhinges enabling adjacent pivoting members to be positioned in closeproximity.
 9. The method according to claim 8, wherein step a) includesproviding an anchor post extending upwardly from the substrate betweenthe first and second supporting regions from which the first hingeextends, and providing a gimbal ring extending from the first hinge fromwhich the second hinge extends.
 10. The method according to claim 8,wherein step a) includes providing the first and second electrodes witharms extending on either side of said second hinge.
 11. The methodaccording to claim 8, wherein step a) includes providing the pivotingmember with an offset section along the first axis adjacent to the firstand second hinges; and wherein step c) includes providing the thirdelectrode beneath the offset section.
 12. The method according to claim8, wherein step b) includes providing the first electrodes with an upperstep proximate the first and second hinges, and a lower step below anouter free end of the first supporting region.
 13. The method accordingto claim 1, wherein step b) includes providing a ground plane extendingaround the first electrode at a level higher than the first electrodedefining a substantially vertical surface therebetween, wherein thesubstantially vertical surface comprises a dielectric surface forisolating the first electrode from the ground plane.
 14. The methodaccording to claim 1, further comprising after step c) the steps of: i)launching an input optical beam into a switch including the MEMs devicevia an input port; ii) dispersing the optical beam into a plurality ofwavelength channels; iii) directing one of the wavelength channels atthe pivoting member in the first position for directing the onewavelength channel at a first output port; iv) executing steps d) and e)to rotate the pivoting member into the second position for directing theone wavelength channels at a second port; whereby the pivoting memberbypasses the third position to avoid directing the one wavelengthchannel into a third port positioned between the first and second ports.15. The method according to claim 14, wherein step i) includesseparating the input optical beam into first and second sub-beams havingthe same polarization; and wherein step iii) includes directing firstand second sub-beams of the one wavelength channel at first and secondsupporting regions, respectively, of the pivoting member.
 16. The methodaccording to claim 14, further comprising directing the input opticalbeam at a concave mirror having optical power between steps i) and ii);and directing the one wavelength channel at the concave mirror betweensteps ii) and iii).